Field of Application
The present invention relates to a vehicle-installed apparatus for use in estimating the position of a vehicle based on variations in a condition of reception of radio waves.
Description of Related Art
Position measurement systems are known for estimating the current position of a vehicle along a travel locus (a path traced out by the vehicle as it moves along a route) by using a relationship between positions along the travel locus and variations in reception condition of radio waves received by an apparatus of the vehicle.
An example of such a position measurement system is described in Japanese patent publication No. 2013-257306, designated in the following as reference document 1. With that system, reference information is derived beforehand which relates successive positions reached by a motor vehicle along its travel locus to corresponding values of RSSI (received signal strength indication) of radio waves received at these positions, transmitted from a base station. Thereafter when a vehicle of the same type advances along a similar travel locus (i.e., along the same road and same travel direction), an apparatus of the vehicle receives the radio waves and generates information relating the RSSI variation to successive increments of distance traveled by the vehicle. That relationship information is compared with the reference information, for use in estimating positions reached by the vehicle along its route.
However with the technology of reference document 1, it is not possible to estimate the position of the vehicle relative to the reference travel locus with respect to a direction at right angles to the reference travel locus, and thereby detect changes in the vehicle position in a direction at right angles to its current advancement direction.